Placement was too carefully postured in the one's Ive read about. Most were animal and votive offerings. Bones and carvings. Most didnt have the tree. I remember another book saying they weren't used as wells, but can't remember why they knew this now. If I google without "celtic" with just shaft burials, I'm getting links to other cultures too.
I dont mind a bit of artistic licence, as long as it's stated that's what it is. A "perhaps" or "maybe" thrown into some books might have saved me a lot of bother.